Strong immunity comes at a cost to fertility
8 November 2010, by Tamera Jones
A strong immune system might lead to a longer life, but it could come at the expense of reduced fertility, say scientists.
They found that Soay sheep that are good at fighting off infections live longer, but don't have as much success at producing offspring each year as sheep with weaker immune systems.
The researchers say this may be because a good immune system takes a lot of energy to maintain: if sheep put all their energies into keeping fit and healthy, they're unlikely to have as much energy to produce offspring.
But they say another reason could be that dampening down the immune system during pregnancy is vital, because a strong immune system can lead to miscarriages.
'We found that sheep with weaker immune systems reproduce every year, frequently produce twins, but die early,' says Dr Andrea Graham from the University of Edinburgh and Princeton University and lead author of the study, published in Science.
'On the other hand, sheep that reach around 12 years of age often skip years altogether.'
'People have tried to answer this question before, but there's so little data on wild animals, the odds of seeing a link between immunity and say, reproduction were never going to be good,' adds Graham.
'Our results may help explain why immune systems can vary so much from one person to the next.'
Dr Andrea Graham, University of Edinburgh and Princeton University
At the extreme ends, they found that female Soay sheep with good immune systems can survive for as long as 15 years, while their less healthy sisters sometimes only live for two or three years.
'These sheep live a harsh life. The major causes of death are infection and starvation,' Graham says.
A strong immune system is essential for survival. But the downside is that it can lead to autoimmunity. Autoimmunity occurs when our immune systems start fighting cells or organs in our own bodies; rheumatoid arthritis, type 1 diabetes and coeliac's disease are common examples of autoimmune diseases.
Scientists think autoimmunity may be more common in the developed world, because we live in such a clean environment. 'We don't come across parasites very often, and in the absence of infection, our immune systems might become more prone to attacking 'self',' says Graham. Researchers call this the hygiene hypothesis.
Graham and her co-authors started their study because they wanted to know if autoimmunity exists in wild animals that are usually full of parasites. Data from a project set up 26 years ago, in which scientists collected data ranging from sheep size, how often they mate to how many lambs they produce allowed the scientists to test their ideas.
Researchers in this latest study also collected blood samples from the same sheep – which live on an island called St Kilda off the coast of Scotland – allowing Graham's team to analyse which antibodies the sheep had in their bodies. They found that antibodies associated with autoimmunity do exist in these sheep.
Next, the researchers looked at how autoimmunity is related to how many lambs the sheep produce.
They found that sheep with the most antibodies were more likely to survive harsh winters, but produced fewer offspring each spring. Sheep with fewer antibodies had more lambs each year, but died earlier.
Although there are striking differences in the reproductive success of these sheep for each year, by the end of their lives, sheep with weak immune systems wind up having around the same total number of offspring as those with better infection-fighting abilities.
The researchers think this may explain why immunity can vary so much between different individuals - at the end of the day, there are benefits to having a weak or a strong immune system, which is why neither has died out.
'Our results may help explain why immune systems can vary so much from one person to the next,' Graham says.
Andrea L. Graham, Adam D. Hayward, Kathryn A. Watt, Jill G. Pilkington, Josephine M. Pemberton, Daniel H. Nussey, Fitness Correlates of Heritable Variation in Antibody Responsiveness in a Wild Mammal, Science, published 29 October 2010, doi: 10.1126/science.1194878
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